So, using the Detroit Diesel engine manual I determine I can connect my webasto suction inlet on the output from the heat exchanger. This allows me to suck water from the lowest point on the engine block. Then I decided my pump output should be on the passenger side head on the front so that the water is forced to travel thru both heads and down the block to the cooler.

It doesn't work. I get no coolant flow at all this way. Absolutely ZERO flow. I've done all the tests == my pump is pumping, if I disconnect one end I get plenty of coolant coming out the supply side of my pump and if I open the valve into the head at that point I get lots of coolant coming out of the head. So, the only thing I can determine is I'm getting no flow thru the engine.

Why not?

Is the problem due to connecting my pump supply to the output of the heat exchanger or is it due to connecting my pump output to the the passenger side head at the back (furthest point back)? Where should I connect if I'm only trying to preheat the block? One of my two connections is the problem and I'm hoping someone familiar with the internals of the engine can help me understand what is wrong with the way I have it hooked up.

On my 92 there was a 3/4" pipe plug in the block just a few inches below the t-stat senders on the curbside bank as you look in from the back engine door. This is the output hot water from the engine (positive pressure). I ran my hose from here to the 40,000 btu heat exchanger then through the little 12v circulator pump. The pump takes this water and sends it around the other side of the engine where I found a 3/4" pipe plug on the back side of the engine water pump (street side of engine as you look in through the engine door) (It's on the back side of the pump so you need a mirror to see the pipe plug). This hole was a suction side of the water pump (negative pressure).

To preheat I turn on the 12v pump. The webasto heats the engine in about 15-25 minutes. When running down the road the coolant flows naturally through the system from engine pump action and the 12v pump isn't needed. This provides coach heat while travelling. The system works very, very well.

If your engine setup is the same you should be able to find these plugs. Install isolation valves on each end for maintenance.

Well, I moved the outlet from my heat exchanger to the back side of the water pump. After starting the engine to help clear the air in the webasto heating lines the preheating system now works great! The nice thing also is my flat plate heat exchanger if VERY good at transfering the heat from the webasto heating loop to the engine heating loop. I'm very impressed with how the heat exchanger transfersalmost all the heat to the engine side of the loop. The flat plate heat exchanger was definitely worth the money! Thanks for the feedback Dave! Within 20 minutes of running the engine was hot to the touch. Now I won't have to worry about being able to start the engine during the cold winter months here in the frozen tundra! Speaking of tundra we are supposed to have snow flurries Wednesday night! UGH, I'm not ready for it to be COLD yet!

I don't have my plumbing completed. Still have to install the pump and finish flushing the engine and refill. I think I'm going to pull the thermostats to run the Calgon so I can get circulation through the radiators. With this cold, I'll never get my engine hot enough to open the thermostats.

Please explain Calgon. Is this not a laundry detergent fabric softener?? Will you leave this in your cooling system?? Why?? Thanks for the enlightenment. Rod

Simply put, Calgon is a substitute for the "expensive" buy it from Detroit Cooling system cleaner. Per our local service center C&J Bus Repair Calgon can be used. Our local service guy got this information/tip directly from a contact he had at Detroit Diesel in the engineering area (don't know any more specifics on it). I used Calgon on my bus and it cleaned out a LOT of gunk from my system. I removed the majority of the Calgon by doing two flushes of the cooling system with destilled water. Use at your own risk and all the standard disclaimers apply. It worked for me and I'm glad I did it. The local service center claims to use it on many of his commercial bus customers for years without issue...

Calgon is bath tub water conditioner..... one box is what I used at a cost of under $2.